m(mg)=3 гр
Hcl
m(H2)=?

   3 г                                                             Х г

   Mg        +        2HCl      =      MgCl2     +    H2

n = 1 моль                                               n = 1 моль

М = 24 г/моль                                          М = 2 г/моль

m = 24 г                                                   m = 2 г

3 г Mg      -    Х г Н2

24 г Mg   -    2 г Н2

m(H2) = 3 * 2 / 24 = 0,25 г